1. Field of the Invention
The present invention relates to an LED driver circuit and an LED lighting device using the same.
2. Description of Related Art
Light emitting diodes (LEDs) have features of low current consumption and long service life, expanding their applications not only in display devices but also in lighting fixtures. Most LED lighting fixtures use a plurality of LEDs in order to obtain desired illuminance (see, for example, Japanese Patent Application Laid-open Nos. 2004-327152, 2006-210836, and 2010-93874).
Most ordinary lighting fixtures use a commercial power supply of AC 100 V. Taking into consideration the use of an LED lighting device in place of an ordinary lighting device such as an incandescent light bulb, the LED lighting device is also desired to be configured to use the commercial power supply of AC 100 V similarly to the ordinary lighting device.
Further, dimming of an incandescent light bulb is controlled using a phase-control dimmer (generally called incandescent lighting controller), which is capable of dimming control on power supply to the incandescent light bulb with ease by using a single volume element for turning ON a switching element (typified by a thyristor element or a triac element) at a given phase angle of an AC power supply voltage (see, for example, Japanese Patent Application Laid-open No. 2005-26142). It is known that, even if the phase-control dimmer is used to control the dimming of the incandescent light bulb, flickering or flashing occurs when a low-watt incandescent light bulb is connected to the dimmer, and the dimming cannot be controlled normally.
For dimming control of an LED lighting device using an AC power supply, the phase-control dimmer is desired to be used similarly to the case of dimming control of the incandescent light bulb. Now, FIG. 20 illustrates a conventional example of an LED lighting system capable of dimming control of an LED lighting device using an AC power supply.
The LED lighting system illustrated in FIG. 20 includes a phase-control dimmer 200, an LED driver circuit 300, and an LED load 400 constituted by a plurality of LEDs. When a knob (not shown) of a preset variable resistor Rvar is set to a given position, the phase-control dimmer 200, which is connected to a commercial power supply 100, turns ON a triac Tri at a power supply phase angle corresponding to the set position of the knob. In addition, the phase-control dimmer 200 includes an anti-noise circuit formed of a capacitor C1 and an inductor L1, and the anti-noise circuit reduces terminal noise which is otherwise fed back to a power supply line from the phase-control dimmer 200.
Further, the LED driver circuit 300 includes a full-wave rectifier 1, a switching control circuit 2, and a switching power supply part 3. The switching power supply part 3 includes a switching element SW1, an inductor L2, a diode D1, a capacitor C4, and a current detection resistor R2. The switching control circuit 2 detects a current value flowing through the current detection resistor R2 connected to a source of the switching element SW1, and controls ON/OFF of the switching element SW1, to thereby control the current flowing through the switching element SW1 to be constant.
In this case, when a maximum value of the current flowing through the switching element SW1 is represented by ip, electric power supplied to the LED load 400, the inductor L2, the diode D1, and the capacitor C4 is 0.5×L×ip2×fo (where L is an inductance of the inductor and fo is a switching frequency (e.g., 60 kHz)). It is desired for the LED driver circuit 300 that a current of the LED load 400 be constant. However, the current supplied to the LED load 400 is not constant because of manufacturing fluctuations or temperature fluctuations about control on the inductor L2, the diode D1, the capacitor C4, and the values of ip and fo, resulting in that brightness of the LED load 400 itself is affected by the above-mentioned manufacturing fluctuations or temperature fluctuations. Although fluctuations in brightness of the LED load 400 in a bright state of high brightness are not perceived as a large change to the human eyes, the fluctuations are not negligible when the LED load 400 is in a dark state of low brightness.
In view of the above, conventionally, there is known an LED driver circuit of a type that controls an LED current to be constant (e.g., Japanese Patent Application Laid-open Nos. 2004-327152 and 2010-93874). Such LED driver circuit can stabilize brightness of an LED in a wide dimming range. However, in the type that controls the LED current to be constant, output electric power is made constant in the entire range in which an output voltage of the full-wave rectifier changes, to which an AC voltage with the phase controlled by the dimmer is input. Therefore, as illustrated in FIG. 21, at a timing t1 at which an instantaneous value of the output voltage of the full-wave rectifier becomes high, an input current is reduced to fall below a lower limit of a holding current of current holding means (e.g., triac) provided inside the dimmer, and then the current holding means is turned OFF. Accordingly, there has been a problem that the dimmer may malfunction with high possibility.